Dielectric medium containing tercyclohexyl



April 14, 1964 a. R.- WILSON ETAL 3,129,277

DIELECTRIC MEDIUM CONTAINING TERCYCLOHEXYL Filed May 17, 1961 L DIELECTRIC METAL CONTAINING ELECTRODES Q-TERCYCLOHEXYL METAL FOIL PAPER IMPREGNATED WITH Q-TERCYCLOHEXYL INVENTORS GEORGE L. BALL, I

GLENN R. WILSON BY AGENT United States Patent 3,129,277 nmtsec MEDIUM CONTAINING rnacrctonnxrr The present invention relates to electrical devices and more particularly deals with apparatus comprising a combination of electricity conducting elements and dielectric insulating and encapsulating medium therefor.

The use of a dielectric is required in certain electrical equipment for the purpose of removing heat and of allowing the making of electrical contact in the absence of air or of explosive mixtures. The requirements for such dielectrics are among other things very high resistance to the how of electric currents, high stability to atmospheric conditions at ordinary and at somewhat elevated temperatures, high resistance to thermal decomposition, low coeflicient of thermal expansion, low vapor pressure, low congealing temperature, low inflammability and high flash point. Preferably the dielectric should be a mobile liquid even at relatively low temperatures and should be a good heat transfer agent. A satisfactory dielectric liquid should not sludge or undergo deleterious change during use. For example, when a capacitor is impregnated with a thermally unstable dielectric, under high operating temperatures and direct current voltages, deterioration of the dielectric results in an ever increasing current leakage, a short capacitor life, corrosion of the electrodes, etc. Moreover, in the case of capacitors operating on alternating current, the decomposition sometimes results in an excessive increase in power factor of the dielectric material.

It is, therefore, an object of the present invention to provide a method for the insulation of elements in electrical apparatus wherein the above objectionable properties are either completely eliminated or substantially reduced. An additional object is to provide transformers, capacitors, circuit breakers, cables, switches, fuses, reactors, regulators, and the like, wherein the deleterious effects caused by unstable dielectric material, such as the deterioration of metal parts, paper insulation, etc., is eliminated or materially reduced. Still another object is to provide electrical apparatus having a long, useful life and good dielectric stability. A further object is to provide a means for suppressing corona discharge in capacitors. Still a further object is the provision of a liquid dielectric composition suitable for encapsulating, i.e., potting, electronic circuits or components, which composition resists the heat generated within the electronic circuits, e. g., hot power tubes, and at the same time is impervious to moisture of the atmosphere. An important object of the invention is the provision of encapsulated electrical equipment, for instance, transformers, motor stators, coils, printed circuits, etc.

According to the invention, these and other objects which will be hereinafter disclosed are provided by the following invention wherein there is provided electrical apparatus comprising, in combination, conducting elements disposed in spaced relationship to each other and adapted during operation of the apparatus to have a difierence in electrical potential therebetween, and tercyclohexyl interposed between said elements for insulating one from the other. The insulating agent, i.e., the dielectric medium of the present apparatus is advantageously one wherein the liquid portion of which consists of at least 50% by weight of tercyclohexyl.

3,129,277 Patented Apr. 14, 1964 The presently useful compounds, i.e., 0-, mand ptercyclohexyl of the formula (meta) They are readily obtainable in known manner, e.g., by the hydrogenation of the corresponding terphenyls, i.e., 0-, mor paterphenyl in a solvent such as hexane at a pressure of say, 2700 p.s.i. and over a hydrogenation catalyst such as copper chromite. We have found that electrical apparatus comprising the tercyclohexyls as liquid dielectric or encapsulating medium are not only resistant to deterioration by ordinary atmospheric conditions, e.g., air and moisture, but that they are distinguished by resistance to high temperatures. At operating temperatures which may he, say up to 400 C. excellent dielectric properties are maintained and low dissipation factors are demonstrated.

Evaluation of the dielectric constant and of the dissipation factor of o-tercyolohexyl was conducted as follows: The test apparatus was a high temperature liquid cell used in conjunction with a bridge network. The cell was a concentric cylindrical capacitor made of gold-plated Monel metal using quartz to insulate the electrodes from each other. Connecting lines were commercially available low loss lines to assure rigid reproducibility in the capacitance of the test cells, and adaptors were employed instead of conventional pin connectors. The connecting lines were free of any material, e.g., insulating beads, which might deteriorate or be rendered ineffective by the high temperatures employed for the tests. The temperature measurements were made with an accuracy of :5" C. This limit of accuracy represented the maximum temperature diiference between the inner and outer electrodes of the cell. There was thus determined dielectric constant values of 2.2 8 to 1.97 at kc. over the temperature range of 35 C. to 330 C., and dissipation factor values varying from 0.09% to 0.16% over the same temperature range.

That no substantial variation in dielectric constant results from a change in frequency is shown by the following values obtained with the o-tercyclohexyl at the frequencies and temperatures shown below:

Dielectric Constant, at- Temperature, C.

1 kc. 10 kc. 100 kc.

The very good thermal stability of the o-tercyclohexyl was determined by aging it at 350 C. for 500 hours. Only negligible changes in dielectric constant and dissipation factor were noted.

The dielectric strength of o-tercyclohexyl when employed in electrical apparatus destined for use at high temperatures was determined by modifying the apparatus Employing o-tercyclohexyl with the perature applications, materials, mica, fused foil (about 0.0003" metal foil in known manner.

used in the American Society for Testing Materials (ASTM) Method D87749 for measurement of dielectric breakdown. The modifications were as follows: Spherical electrodes, 05" in diameter and spaced 0.040"i'0.00=01" apart were used instead of flat electrodes with 0.1" spacing. The breakdown voltage was measured by increasing the voltage across the gap at a rate of 500 volts/second until a break occurred. Correlation between the values obtained with the ASTM cell and the presently employed modified cell was arrived at from a long series of experiments. Those determined with the modified cell were very approximately /2 of the values obtained with the unmodified ASTM cell.

modified cell, dielectric strength values on the ASTM scale were found to be 47 kv. at 23 C. and 22.8 at 275 C.

The isomeric tercyclohexyls are of general utility as dielectric media when interposed in electrical apparatus between metallic conducting elements that have a difference in potential during operation of the apparatus. They are valuable as liquid impregnants in capacitor and cable manufacture, as transformer coolant's, in the fabrication of switch gear, etc.

Characteristics of the tercyclohexyl are such as to render them useful 'as impregnants for cellulosic materials, e.g., wood pulp, paper, cotton, cotton fabric, cellulose acetate fibers and textiles, whereby there are obtained 'materials which higher than that 'impregnation 'or possess dielectric constants that may be ofeither the cellulosic material before of the liquid dielectric. However, since the tercyclohexyls are distinguished by resistance to very 'high temperatures, i.e., temperatures which are not generally tolerated by the cellulosic materials, the more profitable utilization of the tercyclohexyls is in connection with the type of materials encountered in high teme.g., ceramic and other refractory silicas, asbestos, highly fluorinated polymers such as Teflon, etc.

For example, a capacitor employing o-tercyclohexyl is made up of alternate layers of metal foil such as aluminum or tin foil separated by mica. Thus a thin, aluminum in thickness) is covered with a paste of powdered mica and o-tercyclohexyl, and another sheet of the foil is placed thereon so that the two sheets of metal foil are separated from each otherby the paste. This is then covered by another layer of the paste and the stack of alternating layers of metal foil and paste thus obtained is Wound into a cylindrical roll. This is placed into a container, and at this point electrical connecting means, e.g., wire or bar conductors, depending upon the size of the assembly, may be connected to the After drying, preferably in a heated vacuum oven, o-tercyclohexyl is added in a quantity sufficient to fill'all interstices. The container is then sealed.

Alternatively, and particularly in the manufacture of large items, sheets of conducting metal are arranged alternately with sheets of mica at spaced distances from each other within the container, one of the isomeric tercyclohexyls is added, and the container is sealed, V

In the manufacture of a transformer, the core with electrical windings or coil, is positionedwithin the housing or tank, lead-in bushings are provided and suitable leads are connected to the coil assemblypandfthe't'ercyclohexyl compound, e.g., o-tercyclohexyl'is added to the housing, immersing the components contained therein.

In a cable suitable for high temperature, the conducting elements are insulated by asbestos felt which has been conditions of operation or storage.

impregnated with m-tercyclohexyl and contained in a casing of a tetrafluorethylene polymer, e.g., Teflon, or another suitable, flexible, heat-resistant material.

The isomeric tercyclohexyls are useful as liquid encapsulating materials for electrical components generally. They are non-hygroscopic, unaffected by air, stable over wide temperature ranges, and non-corrosive to the electrical components. Hence, they serve not only as liquid coolants and insulating media but also as encapsulating or potting means. This insures the continued high quality of the electrical apparatus, even though it has been repeatedly used and stored for long periods of time.

In the manufacture of electrical apparatus there may be employed as potting agents one of the isomeric tercyclohexyls or a mixture thereof. Also, one or more of the tercyclohexyls may be combined with other dielectric agents or potting agents, depending upon the anticipated ,They may be combined with mica or up to 50% by weight of one or more other dielectric media such as certain liquid petroleum hydrocarbons, the partially chlorinated biphenyls, the chlorinated terphenyls, the chlorinated parafiinic hydrocarbons, chlorinated diphenylethane, the tetrahydrofurfuryl ethers of the chlorophenols, trichlorobenzene, etc.

However, for use in the higher temperature ranges, ad- 'mixture of the terphenyls metallic conducting elements disposed in spaced relationship to each other and exhibiting during operation of the apparatus, a diiference in potential therebetween, and tercyclohexyl interposed between said elements and isolating them from each other.

2. A capacitor comprising conducting elements and tercyclohexyl associated with each of the elements and isolating them from each other.

3. A transformer comprising a core with electrical windings positioned in a housing encompassing tercyclo hexyl contained in said housing.

4. Electrical apparatus comprising, in combination, metallic conducting elements disposed in spaced relationship to each other and exhibiting during operation of the apparatus, a difference in potential therebetween, and o-tercyclohexyl interposed between said elements and isolating them from each other.

5. A capacitor comprising conducting elements and o-tercyclohexyl associated with each of the elements and isolating them from each other.

6. A transformer comprising a core with electrical windings positioned in a housing and encompassed by .o-tercyclohexyl contained in said housing.

References Cited in the file of this patent UNITED STATES PATENTS 2,077,429 McMahon Apr. 20, 1937 2,572,808 Jackson Oct. 23, 1951 2,900,345 Heiuinger Aug. 18, 1959 

1. ELECTRICAL APPARATUS COMPRISING, IN COMBINATION, METALLIC CONDUCTING ELEMENTS DISPOSED IN SPACED RELATIONSHIP TO EACH OTHER AND EXHIBITING DURING OPERATION OF THE APPARATUS, A DIFFERENCE IN POTENTIAL THEREBETWEEN, AND TERCYCLOHEXYL INTERPOSED BETWEEN SAID ELEMENTS AND ISOLATING THEM FROM EACH OTHER. 